1. Field of the Invention
The present invention relates to light-transmitting adhesive compositions having high heat conductivity, high volume resistivity and sufficiently high dielectric and mechanical strength.
Adhesive compositions of this type can be used for joining the elements of optical systems and hermetically sealing solid laser units.
2. Description of the Prior Art
Adhesive compositions have found a wide and successful application in today's industry. Therefore, in most cases where a necessity to use adhesives arises, the technical problem can be successfully solved by using the materials available in the market and conventional methods of compounding and testing the compositions.
Thus there can be solved in most cases the problems of developing, for example, high strength adhesives, dielectric adhesives, heat-resistant adhesive compositions. However, the attempts to apply adhesive compositions made by minor modifications of prior art compositions in manufacturing certain kinds of products prove to be unsatisfactory.
Specifically, adhesive compositions for lasers along with traditionally important properties such as adhesive capacity, mechanical and dielectric strength, adequate heat resistance, must also exhibit high transparency and heat conductivity.
Each of these additional properties together with traditionally required ones can be rather easily imparted to various adhesive compositions. However, it is quite a problem to simultaneously provide for transparency and heat conductivity.
For example, mixtures of epoxy resins are known to be based with at least two epoxy groups in a molecule and hardeners substantially of the polyamide type as adhesive compositions in manufacturing light-transmitting laminates (see CFR Patent No. 1594044).
Adhesive compositions of the type referred to hereinabove have adequate adhesion, mechanical and dielectric strength, are good dielectrics and transmit more than 90% of light.
However, low heat conductivity of these compositions precludes their use for cementing together the elements of optical systems and for hermetially sealing the units of solid lasers.
Heat conductivity can be improved by using adhesive compositions with heat conducting fillers.
Thus, there is known an adhesive composition comprising the following ingredients (wt.%):
low molecular dianic epoxy resin: 100 PA1 polyethylene polyamine as a hardener: 10 to 15 PA1 powdered synthetic diamond as a filler: 200 to 250 PA1 (cf. USSR Inventor's Certificate No. 306,161). PA1 low molecular dianic epoxy resin: 20.0 to 50.0 PA1 the mixture of phenyl-glycidyl ether and aliphatic epoxy resin in the weight ratio of (0.5 to 1.5):1: 2.0 to 15.0 PA1 polyethylene polyamine: 2.0 to 8.0 PA1 finely divided aluminium oxide modified with chromium oxide: 25.0 to 60.0 PA1 preparing a mixture of phenyl-glycidyl ether and aliphatic epoxy resin taken in the weight ratio of (0.5-1.5):1, used as a plasticizer; PA1 heating the low molecular dianic epoxy resin at a temperature of from 40.degree. to 45.degree. C. for 2 to 3 hours to remove volatile impurities and reduce the viscosity; PA1 drying finely divided aluminium oxide modified with chromium oxide at a temperature of from 80.degree. to 85.degree. C. for 2 to 3 hours, used as a filler; PA1 proportioning the above-indicated ingredients according to predetermined quantities; PA1 blending the plasticizer and low-molecular dianic epoxy resin; PA1 introducing the filler into the thus-obtained mixture and additional stirring, under a visual control of the mixture homogeneity; PA1 measuring the predetermined amount of polyethylene polyamine, used as a hardener; PA1 introducing the hardener into the mixture composed of the above mentioned ingredients and additionally stirring the mixture until a homogeneous adhesive composition is obtained. PA1 relative viscosity at a temperature of 42.degree..+-.2.degree. C. determined by the time of a 50-ml sample outflow from a vessel with a cylindric wall and tapered bottom (the angle at the cone vertex is 101.degree.41') provided through the centerthereof with a nozzle 5.4 mm in diameter: max 15 min PA1 lifetime--at a temperature of 42.degree..+-.2.degree. C.: min 30 min PA1 at room temperature of (18.degree.-20.degree. C.): min 50 min PA1 coefficient of thermal conductivity, W/m.multidot..degree.K. PA1 light transmission, % PA1 volume resistivity, Ohm.multidot.cm: min 10.sup.13 PA1 tensile strength, kgf/cm.sup.2 : min 370 PA1 adhesion to solid laser optical system materials, kgf/cm.sup.2 : min 180 PA1 dielectric strength, kV/mm: min 10
The term "low molecular dianic epoxy resin" is used here and hereinafter to denote epoxy resin which is obtained by condensation of diphenylol propane and epichlorohydrin in the presence of an alkali, has not less than 18% of epoxy groups (as determined by the interaction of the resin with hydrochloric acid), is liquid at room temperature, and light yellow to light brown in colour.
The term "polyethylene polyamine" is used here and hereinafter to denote a hardener obtained by the interaction of dichloroethane and an aqueous ammonia solution and having the form of an oily light- to dark brown liquid viscous at room temperature with a density of from 1.00 to 1.04 g/cm.sup.3 and a maximum amine nitrogen content 22%.
The above composition while having adequate adhesion, mechanical and dielectric strength, high volume resistivity and heat conductivity within the range of from 2.0 to 2.2 wt/m.multidot..degree.K. is inadequately transparent. Its luminous transmissivity in the wavelength working range of lasers with active elements from ruby, garnet, sapphire is not more than 25%. Moreover, the viscosity of the described adhesive composition is rather high, which makes its preparation and use more difficult.
It is the main object of the present invention to simultaneously provide for both high heat conductivity and transparency of adhesive composition.
It is another object on the present invention to simplify the preparation and use of adhesive compositions.